NM400 is a high-strength wear-resistant steel plate with a nominal hardness of 400 HBW, widely used in mining and earthmoving equipment. Its name follows the “NM” (N耐磨) Chinese standard (GB/T 24186), where the number indicates the Brinell hardness range. NM400 provides an excellent balance of high hardness, good toughness, and reasonable formability, making it suitable for components subject to severe abrasive wear, gouging, and impact. In the mining industry, NM400 is applied to dump truck bodies, chutes, hoppers, crusher liners, and conveyor transfer points. The high hardness resists scratching and gouging from sharp ores such as iron ore, copper ore, and granite, significantly extending service life compared to ordinary structural steel (e.g., Q345B). Field reports indicate that NM400 liners last three to five times longer than AR360 plates under similar conditions.
For earthmoving equipment, NM400 is commonly used for loader buckets, excavator buckets, bulldozer blades, scraper bowls, and grader moldboards. The plate’s resistance to abrasive wear reduces downtime for repairs and replacement, directly improving productivity. In heavy-duty applications, NM400 is also used as wear strips or overlay backing plates on crusher jaws and hammers. Despite its high hardness, NM400 can be cold-formed to moderate radii. The minimum recommended bending radius for NM400 is typically three to five times the plate thickness when bending transverse to the rolling direction, and five to seven times when bending parallel to the rolling direction. For complex shapes, hot forming at 200–300°C may be used, but never above the tempering temperature (approximately 550°C) to avoid softening.
Welding of NM400 requires careful procedure development. The high carbon equivalent (CEV usually 0.50–0.60) leads to a risk of hydrogen-induced cracking in the heat-affected zone. Therefore, low-hydrogen welding processes (e.g., GMAW with ER80S-G or SMAW with E8018-G electrodes) are mandatory. Preheating is essential for thicknesses above 10 mm; typical preheat temperatures range from 150°C to 250°C, depending on plate thickness, hydrogen level, and restraint. Interpass temperature should not exceed 300°C to avoid excessive heat input. Post-weld heat treatment is generally not required, but if the component will be subjected to dynamic loads, a stress relief at 550–580°C may be applied. Hardness testing of the weld joint should show that the heat-affected zone does not exceed 400 HBW to prevent premature cracking.
In mining trucks, NM400 liners are often used as removable wear packages. The liner plates are attached using countersunk bolts or plug welds to allow easy replacement. For high-impact areas, such as the truck floor where large rocks are dumped, a thicker NM400 plate (20–30 mm) is used, sometimes combined with a resilient backing plate to absorb impact energy. In chutes and hoppers, NM400 liners are arranged in overlapping patterns to prevent material from reaching the base structure. The smooth surface of NM400 also promotes material flow, reducing hang-ups and blockages.
Cost-effectiveness is a major driver for NM400 adoption. While its upfront cost is two to three times that of standard carbon steel, the extended wear life and reduced maintenance downtime result in lower total cost of ownership. For mines operating 24/7, switching from Q345B to NM400 can reduce annual wear parts budget by 40–60%. Overall, NM400 has become the industry standard for heavy abrasion environments, offering a reliable, weldable, and formable solution for mining and earthmoving equipment.
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